Photographic exposure timers providing compensation for supply-voltage variations

For blue-sensitive photographic materials the effective intensity of light from a tungsten-filament lamp is roughly proportional to the fifth power of the applied voltage. To compensate for variations in voltage, electronic timing circuits with simple circuit elements have been developed to effect exposure intervals which are inversely proportional to the fifth power of the supply voltage. With a timer of this type, provided that it is connected to the same supply as the lamp, the exposure (intensity × time) given will be substantially independent of the supply voltage. All the circuits described include RC timing elements, and exposures are adjusted by varying the resistance. For moderate supply-voltage fluctuations sufficient compensation can be provided by varying, in accordance with the supply voltage, either the charging potential, the bias of the relay valve or the initial charge on the condenser. Wider voltage ranges can be covered by using two of these methods simultaneously. A circuit is described which is applicable to any a.c. supply within the range 100¿250 volts. The exposure time at the nominal supply voltage is continuously adjustable from 1 sec to 1 min or more, and voltage deviations of from +15 to ¿20% from the nominal value do not change the intensity-time product by more than ±5%.